The invention relates generally to semiconductor lasers (laser diodes), and more specifically to techniques for varying the wavelength of such lasers in connection with use in fiber optic communications systems.
It is well established that multiple optical communications channels can be optically multiplexed onto a single optical fiber by a technique known as wavelength division multiplexing (WDM). Multiple lasers, each at a different wavelength from the others, are modulated in accordance with respective information patterns, and the modulated light from each laser is output on a respective fiber segment. The light from all the fiber segments is combined onto a single fiber by a wavelength division multiplexer. Light at the other end of the single fiber is separated onto individual fiber segments by a wavelength division demultiplexer, and the light on the individual fiber segments is demodulated to recover the original information patterns.
As the demand for bandwidth has exploded, due in large part to the growth of the Internet and data communications, additional demands are made on the fiber optic technology. A relatively new technology, called dense wavelength division multiplexing (DWDM), is being deployed to expand the capacity of new and existing optical fiber systems. The improvements include providing more wavelength channels, and where possible, increasing the bit rate on each channel.
As is well known, typical single-mode fiber optics communications are at wavelengths in the 1300-nm and 1550-nm ranges. The International Telecommunications Union (ITU) has defined a standard wavelength grid having a frequency band centered at 193,100 GHz, and other bands spaced at 100 GHz intervals around 193,100 GHz. This corresponds to a wavelength spacing of approximately 0.8 nm around a center wavelength of approximately 1550 nm, it being understood that the grid is uniform in frequency and only approximately uniform in wavelength. Implementations at other grid spacings (e.g., 25 GHz, 50 GHz, 200 GHz, etc.) are also permitted.
Since a given fiber in a communications system may need to carry as many as 80 closely spaced wavelengths, it will be appreciated that this translates to a need to provide lasers at all the needed wavelengths. As the bit rates increase and the wavelength channels become more closely spaced, crosstalk becomes an increasing problem. Thus the need to control the lasers"" output wavelengths has become more critical.
The present invention provides flexible and cost-effective optical sources suitable for use in the demanding environment of WDM and DWDM communications. The sources are easily configured for a variety of applications.
In short, an optical source in accordance with one aspect of the invention includes a laser device configured to emit light over a first band of wavelengths, and a waveguide positioned to receive light emitted by the laser device. The waveguide is formed with a grating, and a portion of the waveguide and the grating at least partially define an external cavity. The grating limits light exiting the external cavity in a particular direction (normally away from the laser device) to a second band of wavelengths that is narrower than, and contained within the first band of wavelengths. In particular embodiments, the first band of wavelengths is defined by a semiconductor laser""s optical amplification (SOA) gain spectrum peak while the second band of wavelengths is defined by the grating peak. The grating peak can be one or two orders of magnitude narrower than the SOA gain peak.
The location of the second band of wavelengths depends on a set of one or more properties of the waveguide and grating, and a mechanism is provided for controllably changing at least one of the set of properties so as to change the location of the second band of wavelengths accordingly. This provides a tuning range for the second band, which tuning range can be comparable to the width of the first band of wavelengths.
The set of one or more properties may include the grating""s geometric pitch and/or the waveguide""s refractive index in the vicinity of the grating. The refractive index can be controlled by heating or cooling the grating, or applying tensile or compressive stress to the grating. These also have a direct effect on the grating pitch, but to a significantly lesser extent. The temperature control can be effected by direct contact with a heater element or by radiative heating. Active cooling can be effected through direct contact with a cooling element such as a thermoelectric cooler (TEC). Stress can be applied statically, such as by anchoring the waveguide at two spaced points to a substrate material having a purposely different thermal expansion coefficient, and then heating at least the substrate. The stress can also be induced acoustically where the waveguide is positioned at the node of an acoustic cavity.
Accordingly, the mechanism, depending on the embodiment, may include a temperature control element thermally coupled to the waveguide in the vicinity of the grating or to the substrate material, or an acoustic actuator acoustically coupled to the waveguide in the vicinity of the grating. Moreover, if the portion of the waveguide that contains the grating is formed of an electro-optic material, the mechanism can be a voltage generator applied to electrodes formed on or near the electro-optic portion of the waveguide.
The grating itself can be a permanent structure or a tunable acousto-optical filter (where periodic changes in refractive index are induced by an acoustic standing wave with nodes spaced along the optical axis). Additionally, the waveguide can be formed of two or more different materials, with the grating formed in a region of one of the materials.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.